1. Field of the Invention
The present invention relates to a focus control apparatus installed in a reproduction apparatus which reproduces data from a multilayer optical recording medium.
2. Description of Related Art
One way to improve the recording density of a disk is multiplexing of information in a direction perpendicular to the disk's surface. A multilayer optical disk is capable of implementing such perpendicularly multiplexed information recording. With a double-layer optical disk in use, for example, first and second layers are formed with a spacer region in between as shown in FIG. 1, and the first layer closer to the light irradiated surface of the disk is made translucent so that light receives the second layer.
In reproduction of such a multilayer optical disk, changing the layer from which recorded information is to be read requires that a focus control apparatus should carry out a focus jump operation to rapidly focus read light on the next reading layer.
Normally, the focus jump operation to shift the focal point of read light from the information recorded surface on one layer to the information recorded surface on another is executed based on zero-crossing detection of a focus error signal which is generated based on the output of a pickup.
Specifically, the pickup is provided with a focus actuator, a read-light emitting optical system, for example, which moves an objective lens for determining the focal point of the optical system along the optical axis to thereby shift the focal point of the read light in a direction perpendicular to the surface of the disk. In the initial stage of the focus jump operation, this focus actuator is supplied with an acceleration signal or a focus jump enable signal to shift the focal point of the read light to a target recording surface. Based on the timing of zero-crossing which is detected one after another from a focus error signal acquired during displacement of the focus actuator that is implemented in response to the acceleration signal, a series of operations of terminating the supply of the acceleration signal, supplying a deceleration signal to the focus actuator to stop the displacement of the focus actuator that has taken place in response to the acceleration signal or stopping supplying the deceleration signal and restarting focus servo on the target recording surface.
In this focus control apparatus for a multilayer optical disk, while the point at which a focus error signal passes a zero-crossing point is detected during a focus jump operation, zero-crossing detection is actually carried out with a level a little apart from the zero level taken as a threshold value in consideration of an influence of a noise component or the like. However, the amount of a change in the level of the focus error signal during a focus jump operation is not always constant due to a difference in reflectance from one layer of the disk to another or some stain or the like on the objective lens. If the threshold value is fixed, therefore, a stable focus jump cannot be expected when the supply of an acceleration signal or deceleration signal is stopped at the zero-crossing timing that has been detected in accordance with the fixed threshold value.